1. Field of the Invention
This invention relates to DC/DC converters used in DC/DC and AC/DC power supplies, and more particularly to single ended, zero voltage switched DC/DC converters.
2. Description of the Prior Art
Single ended DC/DC converters are commonly classified as one of three classical topologies: the boost; the buck; and the buck-boost. These converters comprise various arrangements of a switch, a diode, an inductor and two capacitors. Dual circuits of these topologies also exist wherein two inductors and one capacitor appear.
FIGS. 1-2 illustrate examples of DC/DC converters. FIG. 1 illustrates a non-isolated, single switch buck-boost converter having a MOSFET power transistor Q1, inductor L1, diode D1, and capacitors C1 and C3. FIG. IA illustrates the dual circuit of the buck-boost converter wherein shunt capacitors C1 and C3 are replaced by series inductors L2 and L3, shunt inductor L1 by series capacitor C2, and series switch Q1 by shunt switch Q2. FIG. 2 illustrates an isolated, single switch forward converter having transistor Q3, capacitors C4 and C5, diodes D2 and D3, inductor L4 and transformer T1. FIG. 2A illustrates a forward converter with a second flux reset switch Q4 and capacitor C4.
The performance of any circuit is related to the characteristics of the components used in the design, and many recent advances in technology have improved the characteristics of components. Unfortunately, the realization of ideal or lossless switching cannot be achieved with the simple circuits identified above, since in all cases, the switch is turned on while voltage is impressed across it. Any real device will exhibit capacitance between its terminals, and the energy stored in this capacitance (1/2CV.sup.2) will be dissipated when the device turns on.
Additional reactive elements have been added to the basic circuits, thus creating new classes of converters known as resonant converters. FIG. 3 illustrates a full wave series resonant converter wherein the output is controlled by varying the switching frequency. The resonant converter shown includes transistors Q5 and Q6, capacitors C5, C6, and C7, diodes D5 and D6, inductors L5 and L6, and transformer T5. When operated in an appropriate manner, resonant converters can exhibit either zero current or zero voltage switching, thereby significantly reducing the switching loss. In order to accomplish their task, the reactive components must handle considerable power, sometimes amounting to several times the output power of the converter. This energy, which circulates through the components, can induce new losses which are greater than the decrease in switching loss. In addition the operating voltage and/or RMS current stress on the semiconductor devices is often increased.